I. Field of the Invention
The present invention relates generally to memory devices and particularly to supply voltages for sense amplifiers in a memory device.
II. Description of the Related Art
Memory devices are typically provided as internal storage areas in computers. The term memory identifies data storage that comes in the form of integrated circuit chips. There are currently many different types of memory.
One type is random access memory (RAM). This is typically used as the main memory in a computer system. RAM refers to memory that can be both written to and read from. This is in contrast to read only memory (ROM) that permits data to only be read. Most RAM is volatile meaning that it requires a steady flow of power to maintain its contents. When power is turned removed, the data in RAM is lost.
An electrically erasable programmable read-only memory (EEPROM) is a special type of non-volatile ROM that can be erased a byte at a time by exposing it to an electrical charge. EEPROMs comprise a large number of memory cells having electrically isolated gates (floating gates). Data is stored in the memory cells in the form of a charge on the floating gates. The charge is transported to or removed from the floating gates by programming and erase operations, respectively.
A flash memory is a type of EEPROM that can be erased and reprogrammed in blocks instead of one byte at a time. A typical flash memory device comprises a memory array that includes a large number of memory cells arranged in row and column fashion. Each of the memory cells includes a floating gate field-effect transistor capable of holding a charge. The cells are usually grouped into blocks. Each of the cells within a block can be randomly programmed by charging the floating gate. The charge can be removed from the floating gate by a block erase operation. The data in a cell is determined by the presence or absence of the charge in the floating gate.
The memory cells of at least the EEPROM and flash memory devices are coupled through a bit line (also known as a column) to a sense amplifier. When the particular cell is accessed through the row and column signals, that cell is coupled to one input of the sense amplifier. The other input of the sense amplifier may be connected to a reference voltage (Vref). The difference between the two voltages determines if the cell has been programmed. If the cell to be read is programmed, it has a greater voltage than the reference voltage. If the cell to be read is erased, it has a smaller voltage than the reference cell.
In a traditional sense amplifier or differential amplifier, the positive supply for the circuit is connected directly to the power supply for the integrated circuit, Vcc. Typical 1.8V memory devices have a Vcc range of 1.60V to 2.2V. Typical 1.5V memory devices have a Vcc range of 1.35V to 1.70V. This wide Vcc range can cause several problems with memory devices.
During low Vcc operation, the sense time gets slower as Vcc is reduced. The sense amplifier will eventually stop sensing when Vcc goes below a certain threshold.
Conversely, the sensing time improves as Vcc increases. However, as Vcc increases, the bit line bias to the memory cell also increases. Stressing the bit line with a bias voltage of greater than 0.8V for long periods of time can effectively change its VT. This may show up as a potential read disturb problem. There is a resulting need in the art for a sense amplifier scheme that removes both the low Vcc sensing problems as well as the high Vcc read disturb problem.